5962-9313001MPA_10 查看數據表（PDF） - Analog Devices

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5962-9313001MPA_10

High Speed, Video Difference Amplifier

Analog Devices 

AD830

THEORY OF OPERATION

TRADITIONAL DIFFERENTIAL AMPLIFICATION

In the past, when differential amplification was needed to reject

common-mode signals superimposed with a desired signal,

most often the solution used was the classic op amp based

difference amplifier shown in Figure 24. The basic function

VO = V1 − V2 is simply achieved, but the overall performance is

poor and the circuit possesses many serious problems that make

it difficult to realize a robust design with moderate to high

levels of performance.

R1

R2

V2

R3

V1

VOUT

ONLY IF R1 = R2 = R3 = R4

R4

DOES VOUT = V1 – V2

Figure 24. Op Amp Based Difference Amplifier

PROBLEMS WITH THE OP AMP BASED APPROACH

• Low common-mode rejection ratio (CMRR)

• Low impedance inputs

• CMRR highly sensitive to the value of source R

• Different input impedance for the + and − input

• Poor high frequency CMRR

• Requires very highly matched resistors, R1 to R4, to achieve

high CMRR

• Halves the bandwidth of the op amp

• High power dissipation in the resistors for large common-

mode voltage

AD830 FOR DIFFERENTIAL AMPLIFICATION

The AD830 amplifier was specifically developed to solve the

listed problems with the discrete difference amplifier approach.

Its topology, discussed in detail in the Understanding the AD830

Topology section, by design acts as a difference amplifier. The

circuit of Figure 25 shows how simply the AD830 is configured

to produce the difference of the two signals, V1 and V2, in which

the applied differential signal is exactly reproduced at the

output relative to a separate output common. Any common-

mode voltage present at the input is removed by the AD830.

V1

V→ I

V2

IX

A=1

VOUT

IY

V→I

VOUT = V1 – V2

Figure 25. AD830 as a Difference Amplifier

ADVANTAGEOUS PROPERTIES OF THE AD830

• High common-mode rejection ratio (CMRR)

• High impedance inputs

• Symmetrical dynamic response for +1 and −1 Gain

• Low sensitivity to the value of source R

• Equal input impedance for the + and − input

• Excellent high frequency CMRR

• No halving of the bandwidth

• Constant power distortion versus common-mode voltage

• Highly matched resistors not needed

UNDERSTANDING THE AD830 TOPOLOGY

The AD830 represents Analog Devices first amplifier product to

embody a powerful alternative amplifier topology. Referred to

as active feedback, the topology used in the AD830 provides

inherent advantages in the handling of differential signals,

differing system commons, level shifting, and low distortion,

high frequency amplification. In addition, it makes possible the

implementation of many functions not realizable with single op

amp circuits or superior to op amp based equivalent circuits.

With this in mind, it is important to understand the internal

structure of the AD830.

The topology, reduced to its elemental form, is shown in Figure 26.

Nonideal effects, such as nonlinearity, bias currents, and limited

full scale, are omitted from this model for simplicity but are

discussed later. The key feature of this topology is the use of

two, identical voltage-to-current converters, GM, that make up

input and feedback signal interfaces. They are labeled with

inputs VX and VY, respectively. These voltage-to-current

converters possess fully differential inputs, high linearity, high

input impedance, and wide voltage range operation. This

enables the part to handle large amplitude differential signals; it

also provides high common-mode rejection, low distortion, and

negligible loading on the source. The label, GM, is meant to

convey that the transconductance is a large signal quantity,

unlike in the front end of most op amps. The two GM stage

current outputs, IX and IY, sum together at a high impedance

node, which is characterized by an equivalent resistance and

capacitance connected to an ac common. A unity voltage gain

stage follows the high impedance node to provide buffering

from loads. Relative to either input, the open-loop gain, AOL, is

set by the transconductance, GM, working into the resistance,

RP; AOL = GM × RP. The unity gain frequency, ω0 dB, for the open-

loop gain is established by the transconductance, GM, working

into the capacitance, CC; ω0 dB = GM/CC. The open-loop

description of the AD830 is shown below for completeness.

Rev. C | Page 11 of 20

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